THE WINDS. 
225 
observations with the thermometer had aheady induced meteorol- 
ogists to suspect. 
482. It appears, from what has been said 474), that it is the 
rays of the sun operating upon the land, not upon the water, wliich 
causes the monsoons. Now let us turn to Plate VIIL, and exam- 
ine into this view. The monsoon regions are marked with half- 
bearded and half-feathered arrows ; and we perceive, looking at 
the northern hemisphere, that all of Europe, some of Africa, most 
of Asia, and nearly the whole of North America, are to the north, 
or on the polar side of the northeast trade-wind zone ; whereas but 
a small part of Austraha, less of South America, and still less of 
South Africa, are situated on the polar side of the zone of south- 
east trade-winds. In other words, there are no great plains on 
the polar side of the southeast trade-winds upon which the rays 
of the sun, in the summer of the other hemisphere, can play with 
force enough to rarefy the air sufficiently to materially interrupt 
these winds in their course. But, besides the vast area of such 
plains in the northern hemisphere, on the polar side of its trade- 
wind belt, the heat of which is sufficient (§ 479) to draw these 
trade-winds back, there are numerous other districts in the extra- 
tropical regions of our hemisphere the summer heat of which, 
though it be not sufficient to turn the northeast trade-winds back, 
and make a monsoon of them, yet may be sufficient to weaken 
them in their force, and, by retarding them 480), draw the south- 
east trade-winds over into the northern hemisphere. 
483. Now, as this interference from the land takes place in the 
summer only, we might infer, without appealing to actual observa- 
tion, that the position of these trade-wind zones is variable ; that 
is, that the equatorial edge of the southeast trade-wind zones is 
farther to the north in our summer, w^hen the northeast trades are 
most feeble, than it is in winter, when they are strongest. 
484. We have here, then, at work upon these trade-wind zones, 
a force now weak, now strong, which, of course, would cause these 
zones to vibrate up and down the ocean, and within certain limits, 
according to the season of the year. These hmits are given on 
Plate VIII . for spring and autumn. During the latter season 
these zones reach their extreme northern declination, and in our 
spring their utmost limits toward the south. 
485. The Calm Belts. — There is between the two systems of 
P 
